Temperature dependent near-UV molar absorptivities of glyoxal and gluteraldehyde in aqueous solution

VUV/UV oxidation performance for the elimination of recalcitrant aldehydes in water and its variation along the light-path

Vacuum ultraviolet/ultraviolet (VUV/UV) oxidation using a low-pressure mercury lamp emitting dual wavelengths (185 nm (VUV) and 254 nm (UV)) significantly varies in performance along the light-path (l_P), which has not been fully characterized. Therefore, VUV/UV oxidation in solution was investigated at various l_P in terms of the degradation kinetics and mineralization pathway of representative aldehydes with various alkyl-chain lengths. Oxidative degradation of parent aldehydes with shorter alkyl chains was less efficient, specifically the pseudo-zero-order rate constant (k_obs) of formaldehyde was only 51% of that of propionaldehyde (k_obs = 0.078 μM s^-1). In contrast, the mineralization of aldehydes with longer alkyl chains was less efficient because these aldehydes underwent mineralization into more refractory carboxylic byproducts, e.g., oxalic acid. VUV was mainly absorbed by superficial water (l_P < 0.55 cm), which resulted in highly heterogeneous oxidation in homogeneous water. Thus, k_obs of acetaldehyde dramatically decreased from 0.13 to 0.033 μM s^-1 as the total l_P of solution increased from 1.0 to 3.0 cm. On the basis of mineralization pathways proposed above, an iterative kinetic model was developed to characterize the degradation of parent aldehydes and the formation of carboxylic acids along l_P. This model predicted the VUV/UV oxidaton for the first time by considering the fast diffusion of pollutants by limited diffusion of transient radical species. Thus, it realized the prediction of ^•OH concentration at specific water solution and byproduct evolution within specific water solution in turbulent flow regime, wherein ^•OH was predominantly formed in superficial water-layers wherein ^•OH in water-layers of l_P <0.16 cm and <0.81 cm contributed to 50% and 90% of the total oxidation performance, respectively. This result would help to improve the VUV-UV-reactor design in terms of optimizing the thickness of water-layer and turbulence of water-flow.

Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO₂, and NO₂^-/HNO₂) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO₂ oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Biochemical evolution of dissolved organic matter during snow metamorphism across the ablation season for a glacier on the central Tibetan Plateau

The metamorphism of snow (snowmelt process) has a potential influence on chemical and physical process occurring within it. This study carried out a detailed study on the variation of dissolved organic matter (DOM) in different stages of snowmelt in a typical mountain glacier located at Tibetan Plateau through collecting four different surface snow/ice categories, i.e., fresh snow, fine firn, coarse firn, and granular ice during May to October in 2015. The dissolved organic carbon (DOC) was observed by lost 44% from fresh snow to fine firn and enriched 129% from fine firn to granular ice, reflecting the dynamic variability in DOC concentration during snow metamorphism. The absorbance properties of each snow category are positively correlated with DOC concentration. The result of excitation emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) highlighted the domination of lipid- and protein-like compounds in glacial-derived DOM. The molecular composition of the DOM also exhibited a new N-containing molecular formula (CHON classes) that was enriched during snow metamorphism. This study suggests that snow metamorphism could induce a loss of DOM as well as enrich and modify the DOM.

Multiphase chemistry of glyoxal: revised kinetics of the alkyl radical reaction with molecular oxygen and the reaction of glyoxal with OH, NO3, and SO4- in aqueous solution

The rate constant for the reaction of the hydrated glyoxyl radical (CH(OH)2-C(OH)2(·) with O2 has been determined as k(298) K = (1.2 ± 0.3) × 10(9) L mol(-1) s(-1) at pH 4.8. This experimental value is considerably higher than a widely used estimated value of about k = 1 × 10(6) L mol(-1) s(-1). As the aqueous phase conversion of glyoxal is of wide interest for aqSOA formation, we suggest that the newly determined rate constant should be applied in multiphase models. The formation of the dimerization product tartaric acid has as well been studied. This product is found, however in significant yields only when the oxygen content of the solution is reduced. The formation of dimers from the recombination of alkyl radicals in the atmospheric aqueous phase should hence be treated with great care. Finally, the reactions of the free radicals OH, NO3, and SO4(-) with glyoxal have been investigated and rate constants of k(298) K (OH) = (9.2 ± 0.5) × 10(8) L mol(-1) s(-1), k(298) K (SO4(-)) = (2.4 ± 0.2) × 10(7) L mol(-1) s(-1) and k(298) K (NO3) = (4.5 ± 0.3) × 10(6) L mol(-1) s(-1) were obtained.

Complex chemical composition of colored surface films formed from reactions of propanal in sulfuric acid at upper troposphere/lower stratosphere aerosol acidities

Particles in the upper troposphere and lower stratosphere (UT/LS) consist mostly of concentrated sulfuric acid (40-80 wt %) in water. However, airborne measurements have shown that these particles also contain a significant fraction of organic compounds of unknown chemical composition. Acid-catalyzed reactions of carbonyl species are believed to be responsible for significant transfer of gas phase organic species into tropospheric aerosols and are potentially more important at the high acidities characteristic of UT/LS particles. In this study, experiments combining sulfuric acid (H₂SO₄) with propanal and with mixtures of propanal with glyoxal and/or methylglyoxal at acidities typical of UT/LS aerosols produced highly colored surface films (and solutions) that may have implications for aerosol properties. In order to identify the chemical processes responsible for the formation of the surface films, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and ¹H nuclear magnetic resonance (NMR) spectroscopies were used to analyze the chemical composition of the films. Films formed from propanal were a complex mixture of aldol condensation products, acetals and propanal itself. The major aldol condensation products were the dimer (2-methyl-2-pentenal) and 1,3,5-trimethylbenzene that was formed by cyclization of the linear aldol condensation trimer. Additionally, the strong visible absorption of the films indicates that higher-order aldol condensation products must also be present as minor species. The major acetal species were 2,4,6-triethyl-1,3,5-trioxane and longer-chain linear polyacetals which are likely to separate from the aqueous phase. Films formed on mixtures of propanal with glyoxal and/or methylglyoxal also showed evidence of products of cross-reactions. Since cross-reactions would be more likely than self-reactions under atmospheric conditions, similar reactions of aldehydes like propanal with common aerosol organic species like glyoxal and methylglyoxal have the potential to produce significant organic aerosol mass and therefore could potentially impact chemical, optical and/or cloud-forming properties of aerosols, especially if the products partition to the aerosol surface.

Raman spectroscopy of glyoxal oligomers in aqueous solutions

Raman microscopy and Attenuated Total Reflection infrared spectroscopy were utilized to facilitate investigations of equilibria between various hydrated and oligomeric forms of glyoxal in aqueous glyoxal solution droplets. The assignment of spectra is obtained with the assistance of B3LYP density functional quantum chemical calculations of vibrational wavenumbers, Raman activities, and infrared intensities. Several forms of glyoxal derivatives with similar functional groups, e.g., hydroxyl and dioxolane rings, are found to be present. The absence of a Raman spectral peak corresponding to the vibrational carbonyl stretch provides evidence that both carbonyl groups of a glyoxal molecule become hydrated in solutions of a broad concentration range. The presence of bands corresponding to deformation vibrations of the dioxolane ring indicates that dihydrated glyoxal oligomers are formed in glyoxal solutions with concentrations of 1 M and higher. Under typical ambient temperature and humidity conditions, concentrated glyoxal solution droplets undergo evaporation with incomplete water loss. Our results suggest that formation of crystalline glyoxal trimer dihydrate from concentrated solutions droplets is hindered by the high viscosity of the amorphous trimer and requires dry conditions that could rarely be achieved in the atmosphere. However, crystallization may be possible for droplets of low initial glyoxal concentrations, such as those produced by evaporating cloud droplets.

Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

The complexes of glyoxal (Gly), methylglyoxal (MGly), and diacetyl (DAc) with water have been studied using Fourier transform infrared (FTIR) matrix isolation spectroscopy and MP2 calculations with 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the Gly(MGly,DAc)/H2O/Ar matrixes indicates formation of one Gly...H2O complex, three MGly...H2O complexes, and two DAc...H2O ones. All the complexes are stabilized by the O-H...O(C) hydrogen bond between the water molecule and carbonyl oxygen as evidenced by the strong perturbation of the O-H, C=O stretching vibrations. The blue shift of the CH stretching vibration in the Gly...H2O complex and in two MGly...H2O ones suggests that these complexes are additionally stabilized by the improper C-H...O(H2) hydrogen bonding. The theoretical calculations confirm the experimental findings. They evidence the stability of three hydrogen-bonded Gly...H2O and DAc...H2O complexes and six MGly...H2O ones stabilized by the O-H...O(C) hydrogen bond. The calculated vibrational frequencies and geometrical parameters indicate that one DAc..H2O complexes, two Gly...H2O, and three MGly...H2O ones are additionally stabilized by the improper hydrogen bonding between the C-H group and water oxygen. The comparison of the theoretical frequencies with the experimental ones allowed us to attribute the calculated structures to the complexes present in the matrixes.

Spectroscopic studies of methylglyoxal in water and dimethylsulfoxide

Methylglyoxal is a highly reactive dicarbonyl compound, which reacts in vivo with biological macromolecules and thereby affects their structure and function. These changes are associated with complications during aging, diabetes and Alzheimer's disease as well as with growth inhibition in different tumors. Many enzymes are involved in the metabolism of methylglyoxal, but its true physiological role in metabolism and chemical properties are still obscure. In this study it was shown that methylglyoxal, during the freeze-drying of aqueous solutions, polymerizes into small polymeric structures which are stable in organic media such as dimethylsulfoxide. When re-exposed to water, the polymers are immediately transformed into the monomeric mono- and dihydrate forms of methylglyoxal. By NMR and UV spectroscopy, it was shown that solvent, temperature, and the amount of available water strongly influence the equilibrium of the different forms of methylglyoxal and thereby change its reactivity. 1H and 13C NMR spectroscopy were used to determine the structures of the different monomeric and oligomeric structures of methylglyoxal.